1. Field of the Invention
This invention pertains to solid state light emitters and in particular to light emitting diode (LED) packages that are capable of producing high efficiency light emission with a wider emission pattern from a device with smaller footprint.
2. Description of the Related Art
Incandescent or filament-based lamps or bulbs are commonly used as light sources for both residential and commercial facilities. However, such lamps are highly inefficient light sources, with as much as 95% of the input energy lost, primarily in the form of heat or infrared energy. One common alternative to incandescent lamps, so-called compact fluorescent lamps (CFLs), are more effective at converting electricity into light but require the use of toxic materials which, along with its various compounds, can cause both chronic and acute poisoning and can lead to environmental pollution. One solution for improving the efficiency of lamps or bulbs is to use solid state devices such as light emitting diodes (LED or LEDs), rather than metal filaments, to produce light.
Light emitting diodes generally comprise one or more active layers of semiconductor material sandwiched between oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. Light is emitted from the active layer and from various surfaces of the LED.
In order to use an LED chip in a circuit or other like arrangement, it is known to enclose an LED chip in a package to provide environmental and/or mechanical protection, color selection, light focusing and the like. An LED package also includes electrical leads, contacts or traces for electrically connecting the LED package to an external circuit. In a typical LED package 10 illustrated in FIG. 1, a single LED chip 12 is mounted on a reflective cup 13 by means of a solder bond or conductive epoxy. One or more wire bonds 11 connect the ohmic contacts of the LED chip 12 to leads 15A and/or 15B, which may be attached to or integral with the reflective cup 13. The reflective cup may be filled with an encapsulant material 16 which may contain a wavelength conversion material such as a phosphor. Light emitted by the LED at a first wavelength may be absorbed by the phosphor, which may responsively emit light at a second wavelength. The entire assembly is then encapsulated in a clear protective resin 14, which may be molded in the shape of a lens to collimate the light emitted from the LED chip 12. While the reflective cup 13 may direct light in an upward direction, optical losses may occur when the light is reflected (i.e. some light may be absorbed by the reflective cup due to the less than 100% reflectivity of practical reflector surfaces). In addition, heat retention may be an issue for a package such as the package 10 shown in FIG. 1a, since it may be difficult to extract heat through the leads 15A, 15B.
A conventional LED package 20 illustrated in FIG. 2 may be more suited for high power operations which may generate more heat. In the LED package 20, one or more LED chips 22 are mounted onto a carrier such as a printed circuit board (PCB) carrier, substrate or submount 23. A metal reflector 24 mounted on the submount 23 surrounds the LED chip(s) 22 and reflects light emitted by the LED chips 22 away from the package 20. The reflector 24 also provides mechanical protection to the LED chips 22. One or more wirebond connections 27 are made between ohmic contacts on the LED chips 22 and electrical traces 25A, 25B on the submount 23. The mounted LED chips 22 are then covered with an encapsulant 26, which may provide environmental and mechanical protection to the chips while also acting as a lens. The metal reflector 24 is typically attached to the carrier by means of a solder or epoxy bond.
LED chips, such as those found in the LED package 20 of FIG. 2 can be coated by conversion material comprising one or more phosphors, with the phosphors absorbing at least some of the LED light. The LED chip can emit a different wavelength of light such that it emits a combination of light from the LED and the phosphor. The LED chip(s) can be coated with a phosphor using many different methods, with one suitable method being described in U.S. patent application Ser. Nos. 11/656,759 and 11/899,790, both to Chitnis et al. and both entitled “Wafer Level Phosphor Coating Method and Devices Fabricated Utilizing Method”. Alternatively, the LEDs can be coated using other methods such as electrophoretic deposition (EPD), with a suitable EPD method described in U.S. patent application Ser. No. 11/473,089 to Tarsa et al. entitled “Close Loop Electrophoretic Deposition of Semiconductor Devices”.
Another conventional LED package 30 shown in FIG. 3 comprises an LED 32 on a submount 34 with a hemispheric lens 36 formed over it. The LED 32 can be coated by a conversion material that can convert all or most of the light from the LED. The hemispheric lens 36 is arranged to minimize total internal reflection of light. The lens is made relatively large compared to the LED 32 so that the LED 32 approximates a point light source under the lens. As a result, the amount of LED light that reaches the surface of the lens 36 is maximized to maximize the amount of light that emits from the lens 36 on the first pass. This can result in relatively large devices where the distance from the LED to the edge of the lens is maximized, and the edge of the submount can extend out beyond the edge of the encapsulant. Further, these devices generally produce a lambert Ian emission pattern that is not always ideal for wide emission area applications. In some conventional packages the emission profile can be approximately 120 degrees full width at half maximum (FWHM).
Lamps have also been developed utilizing solid state light sources, such as LEDs, in combination with a conversion material that is separated from or remote to the LEDs. Such arrangements are disclosed in U.S. Pat. No. 6,350,041 to Tarsa et al., entitled “High Output Radial Dispersing Lamp Using a Solid State Light Source.” The lamps described in this patent can comprise a solid state light source that transmits light through a separator to a disperser having a phosphor. The disperser can disperse the light in a desired pattern and/or changes its color by converting at least some of the light to a different wavelength through a phosphor or other conversion material. In some embodiments the separator spaces the light source a sufficient distance from the disperser such that heat from the light source will not transfer to the disperser when the light source is carrying elevated currents necessary for room illumination. Additional remote phosphor techniques are described in U.S. Pat. No. 7,614,759 to Negley et al., entitled “Lighting Device.”